Multi-crystalline silicon (mc-Si) grown by directional solidification has attracted much attention in photovoltaic industry because of its low production cost and high throughput. However, the crystal quality deteriorates as the ingot grows taller due to the accumulation of impurities and the generation (multiplication) of dislocations. Because these defects, as well as crystal properties, are affected by grain morphologies and lattice orientations, the control of grain structures is important during crystal growth.
Different from random grain boundaries, special grain boundaries are characterized by particular misorientation and extensive areas of good fit (special grain boundaries are described by a sigma number (1<Σ<29), which is defined as the reciprocal of the fraction of lattice points in the boundaries that coincide between the two adjoining grains on the basis of the coincident site lattice (CSL) model.). Thus, there is low distortion of atomic bonds and relatively little free volume for special grain boundaries and consequently low boundary energy.